Bag-valve resuscitation for treatment of hypotention, head trauma, and cardiac arrest

ABSTRACT

A device for manipulating intrathoracic pressures comprises a compressible bag structure, and an interface member coupled to the bag structure. A one way forward valve is coupled to the bag structure to permit respiratory gas to flow to the patient when the bag structure is compressed. A one way exit valve is employed to allow respiratory gases to be pulled from the person&#39;s airway upon decompression of the bag structure to produce a negative intrathoracic pressure.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 10/660,462,filed on the same date as the present application, entitled “Ventilatorand Methods for Treating Head Trauma, the complete disclosure of whichis herein incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of blood flow, and inparticular to the optimization of blood flow to the heart and brain instates of low blood pressure, head trauma and cardiac arrest. In oneaspect, the invention relates to the intentional manipulation ofintrathoracic pressures to facilitate such blood flow.

Inadequate blood flow can have serious consequences and may result froma variety of conditions. For example, those suffering from low bloodpressure may have inadequate blood flow to the heart and brain. This isespecially true when low blood pressure is the result of blood loss,such as from a serious wound.

Head trauma is generally regarded as the leading cause of morbidity andmortality in the United States for children and young adults. Headtrauma often results in swelling of the brain. Because the skull cannotexpand, the increased pressures within the brain can lead to death orserious brain injury. While a number of therapies have been evaluated inorder to reduce brain swelling, including use of hyperventilation andsteroids, an effective way to treat intracranial pressures remains animportant medical challenge. As described in copending U.S. applicationSer. No. 10/660,462, filed on the same date as the present application,the effects of head trauma may be addressed by decreasing intracranialpressure and increasing cerebral cerebral spinal fluid flow and, to alesser extent, increasing blood flow to the brain. The completedisclosure of this application is herein incorporated by reference.

Those suffering from cardiac arrest lose essentially all blood flow. Ifnot promptly restored, the loss of blood flow can lead to brain injuryor death, among other ailments

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the invention provides a method for enhancing venousreturn to the heart. Such a method may be particularly useful for thosesuffering from cardiac arrest or low blood pressure where venous returnto the heart is critical so that the returned blood may be re-oxygenatedand circulated back through the body. The method may also be useful forthose suffering from head trauma. In such cases, the decreasedintrathoracic pressures cause a reduction in intracranial pressure, anincrease in cerebral spinal fluid flow, and to a lesser extent anincrease in blood now to the brain. Together, this results in decreasedbrain pressures and secondary brain injury. According to the method, apositive pressure breath is delivered to a person. Respiratory gases areextracted from the person's airway following the positive pressurebreath to create an intrathoracic vacuum to enhance venous return to theheart. The steps of delivering positive pressure breaths and extractingrespiratory gases may be repeated to continue the treatment. In someembodiments, the timing of the positive pressure ventilation andgeneration of an vacuum to actively remove respiratory gases from thethorax and thereby decrease intracranial pressures and enhance venousreturn to the heart may be timed with the contraction and/or relaxationof the heart.

In some cases, such as when the person is breathing or during CPR, animpedance threshold valve may also be coupled to the person's airway.The threshold valve prevents airflow to the person's lungs whenattempting to inspire until the threshold valve opens, therebyaugmenting blood flow back to the heart. The threshold valve may beconfigured to open when the negative intrathoracic pressure exceedsabout −6 cmH2O.

In another aspect, a flow limiting valve may be interfaced to thepatient's airway to regulate the pressure and/or flow rate of thepositive pressure breath. In a further aspect, a pressure source and avacuum source may be interfaced to the person's airway to deliver thepositive pressure breath and to extract the respiratory gases.Conveniently, the pressure source and the vacuum source may comprise acompressible bag system. In one aspect, the compressible bag system maybe reconfigured to operate only as a pressure source. For example, thebag system may have a switch that is operated to place the bag system ina ventilate-only mode.

Another feature of the method is that the extracted respiratory gasesmay be exhausted to the atmosphere. In this way, the extracted air isnot re-circulated to the person. In one aspect, the duration oramplitude of the positive pressure breaths or the extraction of therespiratory gases may be varied over time. If needed, the person mayalso be supplied with supplemental oxygen. Also, at least onephysiological parameter of the person may be monitored, and the positivepressure breath or the extraction of respiratory gases may be variedbased on the monitored parameter. Examples of physiological parametersinclude end tidal CO2, oxygen saturation, blood pressure, cardiac outputand the like. Information on the measured parameter may be transmittedto a remote receiver.

In one particular aspect, the respiratory gases may be extracted uponrecoiling of the compressible bag system. The volume of the positivepressure breath may also be measured.

In a further aspect, the intrathoracic vacuum lowers the person'sintrathoracic pressure to about −1 mm Hg to about −20 mm Hg. This may bedone using an intrathoracic vacuum in the range from about −2 mm Hg toabout −60 mm Hg.

The invention also provides a method for treating a person sufferingfrom cardiac arrest. According to the method, a person's chest isrepeatedly compressed. Respiratory gases are prevented or impeded fromflowing to the person's lungs for at least some time between chestcompressions. Periodically, a positive pressure breath is delivered tothe person. Respiratory gases are extracted from the person's airwayfollowing the positive pressure breath to create an intrathoracic vacuumto enhance venous return to the heart. If needed, an impedance thresholdvalve may be coupled to the person's airway to prevent or impede theflow of respiratory gases.

The invention also provides a device for manipulating intrathoracicpressures. The device comprises a compressible bag structure, and aninterface member that is coupled to the bag structure for interfacingwith a person's airway. A one way forward valve is coupled to the bagstructure to permit respiratory gases to flow to the person's airwayupon compression of the bag structure. Also, a one way exit valve iscoupled to the bag structure to permit respiratory gases to be pulledfrom the person's airway upon decompression of the bag structure,thereby producing a negative intrathoracic pressure.

The forward valve and the exit valve may take a variety of forms, suchas a spring loaded check valve, a fish mouth valve, a ball valve, a discvalve, a baffle, a magnetic valve, an electronic valve, and the like. Inone aspect, the bag structure is configured to produce a vacuum in therange from about −2 mm Hg to about −60 mm Hg to produce a negativeintrathoracic pressure in the range from about −1 mm Hg to about −20 mmHg.

Optionally, an impedance threshold valve may be coupled to thecompressible bag structure. The threshold valve is configured to permitrespiratory gases to flow to the person's lungs once a certain negativeintrathoracic pressure is exceeded. In another aspect, a flow limitingvalve may be coupled to the compressible bag to regulate the flow ofrespiratory gases to the patient's lungs upon compression of the bagstructure. Optionally, a switch may be provided for permanently closingthe exit valve.

In a further aspect, an exhaust valve may be coupled to the bagstructure to permit respiratory gases pulled from the person's airway tobe exhausted to the atmosphere. Also, an oxygen source may be used toprovide supplemental oxygen to the person through the interface member.Further, at least one physiological sensor may be operably coupled tothe compressible bag structure to measure at least one physiologicalparameter of the person. A transmitter may be coupled to the sensor totransmit information on the measured parameter to a remote receiver.

In one aspect, a regulation valve may be coupled to the bag structure toregulate the rate of flow of respiratory gases to the person's airwayand/or the pressure of the respiratory gases delivered to the person'sairway. In a further aspect, the bag structure may comprise aventilation chamber that supplies respiratory gases through the forwardvalve upon compression of the bag structure and an expiration chamberthat receives respiratory gases from the person through the exit valveupon decompression of the bag structure. Also, the bag structure mayfurther comprise a venturi system that pulls respiratory gases from theperson's lungs upon decompression of the bag structure. The bagstructure may also constructed of an elastomeric or other spring-likematerial to permit it to decompress.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating one method for enhancing venousreturn to the heart according to the invention.

FIG. 2 is a schematic diagram of one embodiment of a bag-valveresuscitation system according to the invention.

FIG. 3 illustrates a valve arrangement of the system of FIG. 2 alongwith a positive end expiratory pressure valve according to theinvention.

FIG. 4 is a schematic diagram of another embodiment of a bag-valveresuscitation system according to the invention.

FIGS. 5A-5C show three graphics illustrating patterns for delivering apositive pressure breath and extracting respiratory gases according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention may be useful in optimizing blood flow to the heart andbrain in states of low blood pressure, head trauma, cardiac arrest andthe like. For those suffering from head trauma, venous return to thechest may reduce intracranial pressures as described in co-pending U.S.application Ser. No. 10/660,462 filed on the same date as the presentapplication, the complete disclosure of which is herein incorporated byreference.

For those with low blood pressure, the increased circulation may help toincrease their blood pressure. For those in cardiac arrest, bloodcirculation created by the invention serves to help maintain vital organfunctions until resuscitation.

In order to provide such circulation, the invention may utilize anydevice capable of delivering a positive pressure breath followed by thecreation of a vacuum to lower the person's intrathoracic pressure. Thismay be performed with a mechanical ventilator, a ventilation bag and thelike.

One embodiment utilizes a ventilator bag that may be compressed and thenreleased to deliver and then extract air from the person. Such a bag mayinclude a valve system that permits a positive pressure breath to bedelivered when compressing the bag (referred to as the inspiratoryphase) and then immediately pull a vacuum as the bag is released tocause the pressure within the chest to fall less than atmosphericpressure during the expiratory phase.

In some cases, the bag may include a threshold valve as described inU.S. Pat. Nos. 5,551,420; 5,692,498; 6,062,219; 5,730,122; 6,155,257;6,234,916 and 6,224,562, and in U.S. patent application Ser. No.10/224263, filed on Aug. 19, 2002 (“Systems and Methods for EnhancingBlood Circulation”, Ser. No. 10/224,263, filed Mar. 28, 2003 (“DiabetesTreatment Systems and Methods”, Ser. No. 09/966,945, filed Sep. 28, 2001and Ser. No. 09/967,029, filed Sep. 28, 2001, the complete disclosuresof which are herein incorporated by reference. This valve arrangementmay be used to prevent air from entering the person if the pressurewithin the chest is mechanically manipulated to fall (such as during thedecompression phase of manual CPR or ACD CPR) during the expiratoryphase.

In some cases, the rescuer may switch the operation from a “push-pull”ventilator to one that delivers only positive pressure ventilation, suchas is traditional with most ventilator bags( e.g., an AMBU bag).

One reason for pulling the vacuum during the expiratory phase is tolower the intrathoracic pressure within the chest after each positivepressure ventilation. This negative pressure is transferred to the rightheart and lungs, drawing more venous blood back from the extra-thoracicvasculature, and may be used to treat low blood pressure, head traumaand cardiac arrest.

The device may be configured to be hand-held, light weight and portable.As the bag decompresses, it “recharges” itself so that more air isavailable during the next squeeze. Optionally, a foot peddle may beconnected to help develop a greater or more sustained vacuum. It mayalso include a timing device to provide feedback to the rescuers on howoften to ventilate the patient. It may further include a regulator tolimit the amount of pressure that builds up with each positive pressureventilation to prevent stomach insufflation. One example of such aregulator is the SMART BAG®, commercially available from Mediline.

Referring now to FIG. 1, one method for enhancing blood circulation willbe described. In so doing, it will be appreciated that such techniquesmay be used to treat those suffering from head trauma, low bloodpressure, and cardiac arrest, among others.

At step 10, the process may begin by interfacing the appropriateequipment to the person. This may include, for example, a pressure and avacuum source (such as a bag-valve system having a face mask), animpedance threshold valve, a positive pressure flow regulator, one ormore physiological sensors, a transmitter for transmitting measuredsignals to a remote receiver, a metronome or other timing device to tellthe rescuer when to ventilate and/or create a vacuum, an oxygen sourceand the like.

If the person is in cardiac arrest, the rescuer may perform CPR byperforming chest compressions and decompressions as is known in the art.This is illustrated in step 12.

At step 14, a positive pressure breath is delivered to the person. Thisis immediately followed by the extraction of respiratory gases to lowerthe person's intrathoracic pressure as shown in step 16. Steps 12-16 maybe repeated as necessary as shown in step 18. If the person is incardiac arrest, the steps of delivering a breath and extractingrespiratory gases are performed about once for every 5 to 20 chestcompressions. The positive pressure breath may be delivered for about0.5 to about 2.0 seconds while the vacuum may be produced for about 1 toabout 10 seconds. The volume of air delivered may be in the range fromabout 4 ml/kg to about 20 ml/kg. The negative intrathoracic pressurecreated may be in the range From about −1 mmHg to about −20 mmHg. Tocreate the pressure the generated vacuum may be about one to about threetimes this amount.

For those suffering from low blood pressure or head trauma, steps 14 and16 may be continuously performed as long as treatment is needed. Thepositive pressure breath may last about 0.5 to about 3 seconds and havea volume of about 4 ml/kg to about 20 ml/kg. The vacuum may be producedimmediately after the positive pressure breath and last about 1 secondto about 6 seconds. The resulting negative intrathoracic pressure may beabout −1 mm Hg to about −20 mm Hg and may be producing using a vacuumthat is one to about three times this amount. Particular techniques forsupplying the breath and extracting gases are described hereinafter withrespect to FIGS. 5A-5C. Also, it will be appreciated that the vacuum maybe producing using a flow of gases or with no flow, and the time and/oramount of the vacuum may be varied.

As shown in step 20, an impedance threshold valve or other device may beused to prevent or impede respiratory gases from entering the patient'slungs. This may be done, for example, when performing CPR. Duringdecompression after the chest, air is typically drawn into the person'sairway. Using an impedance valve, air is prevented from rushing in untila certain negative intrathoracic pressure is reached. At this time, thevalve opens to permit gases to flow to the lungs. Such techniques aredescribed in the references incorporated herein. For CPR applications,the valve may be set to open when the negative intrathoracic pressureexceeds about −4 cmH₂O to about −15 cmH₂O. Such an impedance valve mayalso be used in non-CPR applications as well when the person inspires.In such cases, the valve may be set to open at about −3 cm H₂O to about−12 cmH₂O.

In step 22, the volume, rate and or pressure of the positive pressurebreath may be regulated. In this way, the patient may be protectedagainst insufflation. In step 24, supplemental oxygen may be supplied tothe patient. This may be supplied based on measured parameter asdescribed below. Also, the oxygen may be delivered to the bag-valvesystem.

In step 26, one or more physiological parameters may optionally bemonitored. The treatments described herein may be varied based on themeasured parameters. Examples of such parameters include end tidal CO₂,oxygen saturation, blood pressure, cardiac output and the like. Otherparameters as well as equipment and sensors that maybe be used aredescribed in copending U.S. application Ser. No. 10/660,462, filed onthe same date as the present application (and incorporated therein byreference) as well as in the other references incorporated herein. Thesemay be coupled to a controller or other computer to record themeasurements, display the measured parameters, recommend or control aspecific treatment and the like.

As shown in step 28, information on the measured parameter may also betransmitted to a remote receiver. This may be over a variety ofcommunication paths or networks, such as wireless networks, cell phones,local area networks, the Internet and the like. This information may beused to evaluate the treatment, monitor the quality of treatment, andcommand a treatment or the like. For example, the information may betransmitted to a hospital or health care facility where a physician mayrecommend how to apply the positive pressure breaths or extract therespiratory gases.

Referring now to FIG. 2, one embodiment of a bag-valve resuscitator 30will be described. Resuscitator 30 may be used in association with anyof the methods described herein. Resuscitator 30 comprises acompressible bag 32 that is divided into a supply chamber 34 and an exitchamber 36. Bag 32 may be constructed of an elastomeric material thatpermits bag 32 to self-expand after it has been compressed. Optionally,an elastomeric material may be placed in one or both of the chambers tofacilitate expansion of bag 32 after it has been compressed. Bag 32 alsoincludes an entrance port 38 and a one-way inflow valve 40. When bag 32is compressed, air, oxygen or other respiratory gases in supply chamber34 are forced through inflow valve 40 and into a conduit 42 where theymay be supplied to a person's airway. Optionally, an interface may becoupled to conduit 42 to couple resuscitator 30 to the patient. Suchinterfaces may include facial masks, endotracheal tubes, and the like.When bag 32 is released, it expands to its normal position. In so doing,inflow valve 40 closes allowing air or other respiratory gases to flowinto chamber 34. Optionally, a flow restrictive device may be used toregulate the flow of air into conduit 42. This may provide a fixedresistance or a variable resistance.

Bag 32 also includes an exit port 44 and a one way outflow valve 46.When bag 32 is compressed, valve 46 closes and gases in chamber 36 mayexit through port 44. As bag 32 expands, valve 46 opens to pullrespiratory gases from the patient's airway. Hence, a positive pressurebreath may be delivered when bag 32 is compressed and gases may beextracted when bag 32 is released. In so doing, the person'sintrathoracic pressure is lowered to pull venous blood back into thechest.

Optionally, one or more sensors 48 may be incorporated into or coupledto resuscitator 30. Examples of sensors that may be used include any ofthose described or incorporated herein. As another option, a timer 50may be coupled to or associated with bag 32. Timer 50 may be a flashinglight, a speaker or the like to indicate when bag 32 should becompressed. This information may be pre-programmed or varied based uponmeasurements from sensor 48.

As shown in FIG. 3, conduit 42 may be modified to include a positive endexpiratory pressure (PEEP) valve 52 for non-breathing patients. This islocated in a non-breather port 54. PEEP valve 52 may be used when theresuscitator bag is switched from one device capable of “pushing andpulling” to one that is locked in the “traditional” positive pressureventilator mode only. However, in some cases, PEEP valve 52 may be usedintermittently, such as every other or every third ventilation cycle.

Resuscitator 30 may also include a switch or a closure valve 56 that maymove to a position that blocks outflow valve 46. In so doing, the “pull”feature is turned off so that respiratory gases are not activelyextracted during the expiratory phase. In another position, valve 56 maybe moved to a position closing non-breather port 54. This option allowsfor standard positive pressure ventilation and for push/pullventilation.

As another option, an impedance threshold valve may be positioned overconduit 42 or anywhere between the bag and the patient. This valve isparticularly useful when performing CPR. When bag 32 is compressed,gases flow through the threshold valve and to the patient to provideproper ventilation. When performing CPR respiratory gases exiting thepatient during compression of the chest pass through the impedance valveand out valve 46. During decompression of the chest, gases are preventedfrom entering the patient's lungs because of the impedance valve. Thisvalve opens when a certain negative intrathoracic pressure is achievedwhen opened gases may enter conduit 42 through valve 40. Such animpedance valve is described in the references incorporated herein.

FIG. 4 illustrates another embodiment of a bag-valve resuscitator 60that comprises a compressible bag 62 that is constructed of anelastomeric material so that it will expand to its original shapefollowing a compression. Bag 62 includes a main ventilation chamber 64that is filled with air or other respiratory gases. When bag 62 iscompressed, air in chamber 643 is directed through a ventilation port66, through a fish mouth valve 68 and into a ventilation tube 70 whereit is supplied to the patient through a patient support 72.

Ventilation chamber 64 is refilled as bag 62 is released and returns toits uncompressed shape. More specifically, as bag 62 decompresses, anegative pressure within main ventilation chamber 64 is produced. Thisopens a one way valve 76 allowing air to flow through a venturi tube 78,through a fish mouth valve 80, through ventilation port 66 and intochamber 64.

Following ventilation, passive expiratory gases from the patient mayflow through patient port 72, into an expiratory chamber 82 and out aone way valve 84.

The generation of the negative intrathoracic pressure occurs during thepassive recoil or decompression of bag 62. More specifically, airflowing through venturi tube 78 creates a venturi effect in tube 86.This creates a negative pressure within a negative chamber 88. In turn,this cases a secondary chamber 90 (which is collapsed) to pen, therebyincluding air flow through a fish mouth valve 92, through a supply tube94 and into secondary chamber 90. Secondary chamber 90 may hold a volumeof about 100 milliliters to about 150 milliliters when filled.

When bag 62 is again compressed, gas stored in secondary chamber 90 isdirected through an exhaust tube 96 and expelled through a fish mouthvalve 98.

Hence, resuscitator 60 may be used in any of the procedures describedherein. Also, resuscitator 60 may include any of the other featuresdescribed in connection with other embodiment described herein, such asflow regulators, threshold valve, sensors, PEEP valves, switches and thelike.

The manner in which positive pressure breaths and the vacuum are createdmay vary depending upon a particular application. These may be appliedin a variety of waveforms having different durations and slopes.Examples include using a square wave, biphasic (where a vacuum iscreated followed by positive pressure, decay (where a vacuum is createdand then permitted to decay), and the like. Three specific examples ofhow this may occur are illustrated in FIGS. 5A-5C, although others arepossible. For convenience of discussion, the time during which thepositive pressure breath occurs may be defined in terms of theinspiratory phase, and the time during which the intrathoracic pressureis lowered may be defined in terms of the expiratory phase. As shown inFIG. 5A, respiratory gases are quickly supplied up to a pressure ofabout 22 mmHg. This is immediately reversed to a negative pressure ofabout −10 mmHg. This pressure is kept relatively constant until the endof the expiratory phase where the cycle is repeated. In some cases, thecycle may go from a push-pull every breath to a push, then push-pullevery other breath or every third breath, i.e. as a 2:1 or 3:1 push:pulloption.

In FIG. 5B, the positive pressure is more slowly applied. When reachinga pressure of about 10 to about 15 mmHg, the pressure is rapidlyreversed to a negative pressure of about −20 mmHg. The negative pressuregradually declines to about 0 mmHg at the end of the expiratory phase.The cycle is then repeated. Hence, in the cycle of FIG. 5B, the positivepressure is reduced compared to the cycle in FIG. 5A, and the negativepressure is initially lower, but allowed to gradually increase. Thetechnique is designed to help reduce a possible airway collapse.

In FIG. 5C, the positive pressure is brought up to about 20 mmHg andthen immediately brought down to about 0 mmHg. The negative pressure isthen gradually increased to about −20 mmHg toward the end of theexpiratory phase. This cycle is designed to help reduce a possibleairway collapse.

The invention has now been described in detail for purposes of clarityand understanding. However, it will be appreciated that certain changesand modifications may be practiced within the scope of the appendedclaims.

1. A method for enhancing venous return to the heart, the methodcomprising: repetitively compressing the patient's chest: delivering apositive pressure breath for about 0.5 seconds to about 2 seconds to aperson suffering from low blood pressure or head trauma; activelyextracting respiratory gases from the person's airway following thepositive pressure breath to create an intrathoracic vacuum to enhancevenous return to the heart, wherein the intrathoracic vacuum lowers theperson's intrathoracic pressure to about −1 mm Hg to about −20 mm Hg;and repeating the steps of delivering positive pressure breaths andextracting respiratory gases.
 2. A method as in claim 1, furthercomprising interfacing an impedance threshold valve to the person'sairway, wherein the threshold valve prevents airflow to the person'slungs when attempting to inspire until the threshold valve opens,thereby augmenting blood flow back to the heart.
 3. A method as in claim2, wherein the threshold valve is configured to open when the negativeintrathoracic pressure exceeds about −7 cmH2O.
 4. A method as in claim1, further comprising interfacing a flow limiting valve to the patient'sairway and regulating the pressure or the volume of the positivepressure breath with the flow limiting valve.
 5. A method as in claim 1,further comprising interfacing a pressure source and a vacuum source tothe person to deliver the positive pressure breath and to extract therespiratory gases.
 6. A method as in claim 5, wherein the pressuresource and the vacuum source comprise a compressible bag system.
 7. Amethod as in claim 6, further comprising reconfiguring the compressiblebag system to operate only as a pressure source.
 8. A method as in claim1, further comprising exhausting the extracted respiratory gases to theatmosphere.
 9. A method as in claim 1, further comprising varying theduration of the positive pressure breaths or the extraction of therespiratory gases over time.
 10. A method as in claim 1, furthercomprising supplying supplemental oxygen to the person.
 11. A method asin claim 1, further comprising monitoring at least one physiologicalparameter of the person and varying the positive pressure breath or theextraction of respiratory gases based on the monitored parameter.
 12. Amethod as in claim 11, wherein the physiological parameters are selectedfrom a group consisting of end tidal CO2, oxygen saturation, bloodpressure and cardiac output.
 13. A method as in claim 11, furthercomprising varying the amplitude of the positive pressure breath or theextraction of respiratory gases.
 14. A method as in claim 6, wherein therespiratory gases are extracted upon recoiling of the compressible bagsystem.
 15. A method as in claim 1, wherein the intrathoracic vacuum isin the range from about −2 mm Hg to about −60 mm Hg.
 16. A method as inclaim 1, further comprising measuring the volume of the positivepressure breath.
 17. A method as in claim 11, further comprisingtransmitting information on the measured parameter to a remote receiver.18. A method for treating a person with low blood pressure or headtrauma who needs assisted ventilation, the method comprising: deliveringa positive pressure breath for about 0.5 seconds to about 2 seconds to aperson suffering from low blood pressure or head trauma; activelyextracting respiratory gases from the person's airway following thepositive pressure breath to create an intrathoracic vacuum to enhancevenous return to the heart, wherein the intrathoracic vacuum lowers theperson's intrathoracic pressure to about −1 mm Hg to about −20 mm Hg;and repeating the steps of delivering positive pressure breaths andextracting respiratory gases.